Referring to FIG. 1, a color CRT 10 generally comprises an evacuated glass envelope consisting of a panel 12, a funnel 13 sealed to the panel 12 and a tubular neck 14 connected by the funnel 13, an electron gun 11 centrally mounted within the neck 14 and a shadow mask 16 removably mounted to a sidewall of the panel 12. A three color phosphor screen is formed on the inner surface of a display window or faceplate 18 of the panel 12.
The electron gun 11 generates three electron beams 19a or 19b, said beams being directed along convergent paths through the shadow mask 16 to the screen 20 by means of several lenses of the gun and a high positive voltage applied through an anode button 15 and being deflected by a deflection yoke 17 so as to scan over the screen 20 through apertures or slits 16a formed in the shadow mask 16.
In the color CRT 10, the phosphor screen 20, as shown in FIG. 2, comprises an array of three phosphor elements R, G and B of three different emission colors arranged in a cyclic order of a predetermined structure of multiple-stripe or multiple-dot shape and a matrix of light-absorptive material surrounding the phosphor elements R, G and B.
A thin film of aluminum 22 overlies the screen 20 in order to provide a means for applying the uniform potential applied through the anode button 15 to the screen 20, increase the brightness of the phosphor screen and prevent from degrading ions in the phosphor screen and decreasing the potential of the phosphor screen. And also, a film of resin such as lacquer (not shown) may be applied between the aluminum thin film 22 and the phosphor screen to enhance the flatness and reflectivity of the aluminum thin film 22
In a photolithographic wet process, which is well known as a prior art process for forming the phosphor screen, a slurry of a photosensitive binder and phosphor particles is coated on the inner surface of the faceplate. It does not meet the higher resolution demands and requires a lot of complicated processing steps and a lot of manufacturing equipments, thereby necessitating a high cost in manufacturing the phosphor screen. And also, it discharges a large quantity of effluent such as waste water, phosphor elements, 6th chrome sensitizer, etc., with the use of a large quantity of clean water.
To solve or alleviate the above problems, the improved process of electrophotographically manufacturing the screen utilizing dry-powdered phosphor particles is developed. U.S. Pat. No. 4,921,767, issued to Datta at al. on May 1, 1990, describes one method of electrophotographically manufacturing the phosphor screen assembly using dry-powdered phosphor particles through the repetition of a series of steps represented in FIGS. 3A to 3E, as is briefly explained in the following (FIG. 3D and FIG. 3E respectively show a developing step and a fixing step described in our copending Korean patent application Serial No. 95-10420 filed on Apr. 29, 1995 and assigned to the assignee of the present invention.)
Prior to the electrophotographic screening process, foreign substance is clearly removed from an inner surface of a panel by several conventional methods. Then, a conductive layer 132, as shown in FIG. 3A, is formed by conventionally coating the inner surface of the viewing faceplate 18 with a suitable conductive solution comprising an electrically conductive material which provides an electrode for an overlying photoconductive layer 134. The conductive layer 132 can be an inorganic conductive material such as tin oxide or indium oxide, or their mixture or, preferably, a volatilizable organic conductive material consisting of a polyelectrolyte commercially known as polybrene (1,5-dimethyl-1,5-diazaundecamethylene polymethobromide, hexadimethrine bromide), available from Aldrich Chemical Co., Milwaukee, Wis., or another quaternary ammonium salt. The polybrene is conventionally applied to the inner surface of the viewing faceplate 18 in an aqueous solution containing about 10percent by weight of propanol and about 10 percent by weight of a water soluble, adhesion promoting polymer such as poly(vinyl alcohol), polyacrylic acid, certain polyamide and the like, and the coated solution is dried to form the conductive layer 132 having a thickness from about 1 to 2 microns and a surface resistivity of less than about 10.sub.8 ohms per square unit.
The photoconductive layer 134 is formed by coating the conductive layer 132 with a photoconductive solution comprising a volatilizable organic polymeric material, a suitable photoconductive dye and a solvent. The polymeric material is an organic polymer such as polyvinyl carbazole, or an organic monomer such as n-ethyl carbazole, n-vinyl carbazole or tetraphenylbutatriene dissolved in a polymeric binder such as polymethylmethacrylate or polypropylene carbonate. The suitable dyes, which are sensitive to light in the visible spectrum, preferably from about 400 to 700 nm, include crystal violet, chloridine blue, rhodamine EG and the like. This dye is typically present in the photoconductive composition in from about 0.1 to 0.4% by weight. The solvent for the photoconductive composition is an organic such as chlorobenzene or cyclopentanone and the like which will produce as little cross contamination as possible between the layers 132 and 134. The photoconductive solution is conventionally applied to the conductive layer 132, as by spin coating, and dried to form a layer having a thickness from about 2 to 6 microns.
FIG. 3B schematically illustrates a charging step, wherein the photoconductive layer 134 overlying the conductive layer 132 is positively charged in a dark environment by a conventional positive corona discharger 136, which moves across the layer 134 and charges it within the range of +200 to +700 volts.
FIG. 3C schematically shows an exposure step, wherein the shadow mask 16 is inserted in the panel 12 and the charged photoconductor is exposed through a lens system 140 and the shadow mask 16, to the light from a xenon flash lamp 138 disposed at one position within a conventional three-in-one lighthouse. Then, the positive charges of the exposed areas are discharged through the grounded conductive layer 132 and the charges of the unexposed areas remain in the photoconductive layer 134, thus establishing a latent charge image in a predetermined array structure. Three exposures are required for forming a light-absorptive matrix with three different incident angles, respectively.
FIG. 3D diagrammatically illustrates the outline of a developing step, as described in the Korean patent application Serial No. 95-10420 cited above. In FIG. 3D, after removing the shadow mask 16, suitably charged, dry-powdered particles such as particular color-emitting phosphor particles or light-absorptive material particles are sprayed by compressed air toward a photoconductive layer 134 through a venturi tube 146 and a nozzle 144bfrom a hopper 148 and attracted to one of the charged or unexposed areas and the discharged or exposed areas depending upon the polarity of the charged particles due to electrical attraction or repulsion, thus one of the two areas is developed in a predetermined array pattern. Below the nozzle 144b, there is provided a discharge electrode 144a such as a corona discharger for charging dry-powdered particles to be sprayed in the nozzle 144b. The light-absorptive material particles for directly developing the unexposed or positively charged areas are negatively charged and the phosphor particles are positively charged for reversely developing the exposed or discharged areas. The charging of the dry-powdered particles may be executed by a triboelectrical charging method using surface-treated carrier beads, as disclosed in U.S. Pat. No. 4,921,767 cited above.
FIG. 3E schematically illustrates a fixing step using a vapor swelling method, as described in the Korean patent application serial No. 95-10420 cited above.
In the fixing step, the surface of polymers contained in the photoconductive layer 134 are dissolved by coming into contact with solvent vapor such as acetone, methyl isobutyl ketone, etc., on the surface of the developed photoconductive layer 134, said dissolved polymers fixing the dry-powdered particles deposited on the developed areas of the photoconductive layer 134.
The fixing step also may be executed by infrared radiation to fix the deposited particles by melting or thermally bonding the polymer components of the particles 21 and the photoconductive layer 134 to the photoconductive layer 134, as disclosed in U.S. Pat. No. 4,921,767 cited above.
The steps of charging, exposing, developing and fixing are repeated for the black matrix particles and the three different phosphor particles. The faceplate panel 12 is baked in air at a temperature of 425 degrees centigrade, for about 30 minutes to drive off the volatilizable constituents of screen including the conductive layer 132, the photoconductive layer 134, the solvents present in both the screen structure materials and in the filming lacquer, thereby forming an screen array of light-absorptive material 21 and three phosphor elements R, G and B in FIG. 2.
The aforementioned process, as disclosed in U.S. Pat. No. 4,921,767 cited above, has one problem that it requires dark environment during performing all the steps since the photoconductive layer is sensitive to the visual light.
Korean patent application serial No. 95-10420, cited above, and U.S. Pat. No. 5,413,885 disclose a method of electrophotographically manufacturing the CRT screen under visible lights or low intensity yellow lights of 577-597nm using a novel photoconductive layer to solve the aforementioned problem. The photoconductive layer is formed by applying a photoconductive solution containing bis dimethyl phenyl diphenyl butatriene as a donor of ultraviolet-sensitive material, and one of trinitro fluorenone (TNF), ethylanthraquinone (EAQ) and their mixture as an acceptor with polystyrene as polymer binder.
The photoconductive solution, which, as described in FIG. 3A, contains the organic polymer or an organic monomer such as n-ethyl carbazole, n-vinyl carbazole or tetraphenylbutatriene dissolved in a polymeric binder such as polymethyl-methacrylate or polypropylene carbonate, and the suitable dyes sensitive to light, or which contains bis dimethyl phenyl diphenyl butatriene and one of trinitro fluorenone (TNF), ethylanthraquinone (EAQ) and their mixture with polystyrene, is applied to the conductive layer 132, thereby the photoconductive layer 134 being formed.
However, since said photoconductive layer 134 has low charge characteristics and the applied potential is limited in order to prevent the damage of the photoconductive layer 134, there are some problems that it takes much time to charge the photoconductive layer 134 with the corona discharger 144a in FIG. 3B and the whole surface of the photoconductive layer 134 is not charged uniformly. Also, further problem is that said bis-1,4-dimethyl phenyl(-1,4-diphenyl(butatriene)) is not volatilized perfectly after burning in the frit step of bulb and 8 or 10 wt. % thereof remains on the screen structure of the panel.
In order to remove the aforementioned problems, it is an object of the present invention to provide a photoconductive solution which has excellent charge characteristics with easy control of charge amount and is completely volatilized after baking.